Potassium nitrate, otherwise known as saltpeter or nitrate of potash, is important in the production of fertilizers, explosives, glass, and numerous other industrial chemicals. It is one of the oldest known "industrial" chemicals. Potassium nitrate has been used on a large scale since around the year 1300, when the Chinese discovered that saltpeter could be combined with sulfur and charcoal to produce the common explosive known as black powder.
The ever growing demand for potassium nitrate for these and other such uses has resulted in a prolonged search for improved potassium nitrate production processes, and various methods have been invented to produce potassium nitrate. For example, large quantities of potassium nitrate are commercially produced by the reaction of potassium chloride with nitric acid in the presence of oxygen, yielding the following overall reaction: EQU 2KCl+2HNO.sub.3 +1/2 O.sub.2 .fwdarw.2KNO.sub.3 +Cl.sub.2 H.sub.2 O.
The potassium chloride and nitric acid must be reacted at 100.degree. C. to produce potassium nitrate, nitrosyl chloride and water as follows: EQU 3KCl+4HNO.sub.3 .fwdarw.3KNO.sub.3 +NOCl+Cl.sub.2 +2H.sub.2 O.
The nitrosyl chloride is then oxidized to chlorine and nitrogen peroxide, NO.sub.2, with nitric acid. See Chemical Process Industries, 4th Ed., Shreve and Brink, McGraw-Hill, Inc., New York (1977), pp. 272-273.
Smith et al, in U.S. Pat. No. 2,963,345, herein incorporated by reference, disclose a process for producing potassium nitrate, which involves agitating solid particulate potassium chloride with liquid nitrogen peroxide under anhydrous conditions at a temperature of 15.degree. C.; excess nitrosyl chloride vapors produced by the reaction are continuously withdrawn to maintain the reaction. Potassium nitrate and unreacted potassium chloride are then separated by addition to a brine that contains dissolved potassium nitrate and potassium chloride; the brine solution is heated to about 85.degree. C. to dissolve the potassium nitrate, but not the solid particles of potassium chloride. The solid particles of potassium chloride are then separated by filtration.
Large volumes of potassium nitrate are also produced by the reaction of sodium nitrate with potassium chloride, the overall reaction being: EQU KCl+NaNO.sub.3 .fwdarw.KNO.sub.3 +NaCl.
This process requires that potassium chloride be dissolved in a hot solution of sodium nitrate; upon heating, sodium chloride crystals are formed. The hot potassium nitrate solution is then run through the sodium chloride crystals forming at the bottom of the reaction vessel. However, a mixture of potassium nitrate and sodium chloride is formed, so additional processing operations are required to separate potassium nitrate.
Lehto, in U.S. Pat. No. 3,983,222, herein incorporated by reference, discloses a continuous process for producing potassium nitrate, which includes the steps of extracting nitrate from aqueous solutions with an organic amine salt dissolved in an organic solvent, separating the organic phase containing the extracted nitrate from the aqueous phase, and stripping the organic base with a potassium salt stripping solution having a pH of at least 0.5. The stripping solution also contains nitrate ions and potassium ions with the concentration of potassium nitrate maintained high enough to induce crystallization of potassium nitrate from the stripping solution continuously.
Dotson et al, U.S. Pat. No. 4,465,568, herein incorporated by reference, uses an electrolytic process to produce chloride free mixtures of sodium nitrate and potassium nitrate.
All of the prior art processes for producing potassium nitrate are expensive or difficult to perform. Processes that utilize nitric acid at elevated temperatures require specially constructed equipment to handle the highly corrosive reactants, and further, elevated reaction temperatures require high energy inputs. Other prior art processes suffer from low yields of potassium nitrate or an impure product, while others involve the use or production of nitrogen peroxide, which is toxic, and poses a pollution problem.
Thus, there is a need for an inexpensive and continuous process for producing large quantities of potassium nitrate at ambient temperatures. There is also a need for a potassium nitrate production process which does not corrode reaction vessels, and thereby require expensive corrosion resistant construction materials. Further, there is a need for a safe potassium nitrate production process which produces by-products which are easy to handle, and dispose of.
Reaction of potassium chloride with nitric acid to produce potassium nitrate via ion exchange has not been attempted, since a potential hazard exists in the use of nitric acid in ion exchange operations. There have been several accidents involving the use of nitric acid as a regenerant or elution agent with ion exchange resins. Nitric acid is a powerful oxidizing agent, and the reaction of nitric acid with organic ion exchange resins can result in a serious fire or explosion. Further, while the use of dilute solutions of nitric acid may reduce the risk of explosion or fire, the presence of metals, such as copper, and absorbed organic solutes in any system containing nitric acid can catalyze an uncontrolled reaction. Even in dilute solutions, nitric acid is believed to have a negative effect on the useful life of exchange resins.
At dilute nitric acid concentrations, larger volumes of resin are needed, with the resulting increase in cost, without a substantial decrease in the perceived potential for a fire or explosion. The necessity of using large volumes of expensive resin to achieve reasonable yields of product further discouraged the use of ion exchange to produce potassium nitrate.
The production of potassium nitrate by passing a neutral nitrate salt through a cationic exchange resin was also not believed practical, since cationic exchange resins have an equal affinity for potassium and other monovalent ions. Divalent ions, such as calcium, make regeneration of such a column difficult, since large quantities of potassium are necessary to displace calcium bound to the resin. Yet, provided the aforementioned problems can be overcome, production of potassium nitrate via ion exchange offers a simple, low cost and efficient alternative to the prior art methods.